1. Field of the Invention
This invention is related in general to liquid suspensions of small solid particles and, in particular, to a method for decreasing the apparent density of particulates dispersed in liquids, so as to improve their ability to remain in suspension.
2. Description of the Prior Art
Many commercial products consist of solid particles suspended in a liquid carrier for preservation and for ease of application. These products are not true colloids, inasmuch as the solid particles tend to separate out of the suspension within a relatively short period of time after mixing, thus creating problems of uniformity during application of the product. Therefore, it is necessary to stir the mixture vigorously to create a homogeneous suspension prior to, and sometimes even during, use. Examples of such products are some paints and pesticides that consist of solid active ingredients mixed in a liquid carrier. Therefore, this disclosure is presented with particular reference to pesticidal material for convenience, but it is not to be interpreted to be so limited in scope.
During commercial application, pesticides are usually pumped from a retaining tank in a moving vehicle through a system of articulated delivery pipes and spray nozzles. Typical pesticide formulations consist of suspensions that separate into two distinct phases rather rapidly, thus allowing the heavier particulate phase to settle at the bottom of the retaining tank and produce a non-homogeneous mixture. Thus, these products need to be mixed thoroughly before and during use to ensure uniform application and avoid clogging of the delivery system.
In order to increase the stability of solids suspended in liquids (sol-type colloidal suspensions), chemicals such as electrolytes may be added in the dispersing medium to provide attractive forces between the two phases and render the suspension intrinsically stable (that is, to create a lyophilic colloidal system). Another option is to reduce the size of the particulates because it is known that smaller particles form a more homogeneous mixture. As the size of the particles is reduced, the effects of surface phenomena become more important than the difference in density between the liquid and solid phases and gravity alone is not sufficient to cause separation. Stable colloidal behavior is usually associated with particles sizes ranging from 10.sup.-5 to 10.sup.-7 cm.
Pesticides typically contain larger active particles, in the 10.sup.-2 -10.sup.-5 cm range, which tend to settle out of dispersion in a few minutes. The particle size of the solid phase in these products cannot normally be altered without affecting the product itself and the addition of chemicals increases the cost of the product. Therefore, neither option is desirable. Accordingly, there still exists a need for a method of improving the stability of solid suspensions without affecting the chemical balance or the physical characteristics of the dispersed phase.